Interest in contaminant transport has increased due to concerns about the fate of radionuclides in the subsurface environment, particularly with regard to the possible contamination of soil and groundwater by radioactive waste disposal both in near-surface and underground repositories. Although significant progress has been made, the quantitative description of contaminant transport in geological formations remains a challenging and active research area. This paper presents the estimation of transport parameters for a non-sorbing radionuclide (tritium) and a sorbing radionuclide (cesium-137) by fitting the experimental breakthrough curves with a Convection-Dispersion Equation (CDE) model. These experiments were performed in order to estimate the Barremian limestone dispersivity.

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The estimation of the radionuclide transport parameters using the cde model

Lucrarea descrie evaluarea timpului de transfer pentru 60Co si 137Cs in straturile geologice ale amplasamentului Saligny care este selectat pentru construcţia unui depozit de deseuri radioactive. Sunt prezentate aspectele geologice ale amplasamentului Saligny necesare pentru evaluarea timpului de transfer al radionuclizilor. In lucrare este prezentat modelul conceptual si modelul matematic asociat care descrie transferul radionuclizilor din sistemul de depozitare deseuri radioactive in acvifer, si este utilizat pentru calcularea variaţiei concentraţiilor 60Co si 137Cs in fiecare compartiment al sistemului de depozitare. Timpii de transfer al 60Co si al 137Cs sunt derivaţi din diferenţa dintre timpul când un radionuclid atinge concentraţia maxima in doua compartimente adiacente. Timpii de transfer al 60Co si al 137Cs sunt comparaţi cu timpii de injumataţire al radionuclizilor in scopul evaluarii rolul straturilor geologice ale amplasamentului sa intârzie transferului radionuclizilor si in acelasi timp sa permita dezintegrarea naturala a radionuclizilor. This paper describes the assessment of 60 Co and 137 Cs transfer times in the geological layers of Saligny site which is selected for the construction of a radioactive waste repository. The geological aspects of Saligny site necessary to assess the transfer time of radionuclides are presented. The conceptual model of the repository as well as the associated mathematical model which describes the transfer of radionuclides from the radioactive waste disposal system to the aquifer is used to calculate the variation of 60 Co and 137 Cs concentrations into each compartment of the disposal system are presented in the paper. The 60 Co and 137 Cs transfer times are derived from the time difference of a radionuclide reaching the highest pick value of concentration into two adjacent compartments. The 60 Co and 137 Cs transfer times are compared with the half life of radionuclides in order to asses the role of the geological layers of the site in delaying the transfer of radionuclides, allowing as well the natural decay of radionuclides.

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Estimation of radionuclide transfer times in the geological layers of saligny site

The Saligny Site, situated in the vicinity of the Cernavoda Nuclear Power Plant in Romania, has been chosen as a repository site for placement of future Low and Intermediate Level Nuclear Waste. Scientific investigations are on going to help assess the long-term performance of the nuclear waste repository and include the development of a site conceptual model, geochemical characterization of the proposed liner material and the geologic formations surrounding the repository. Geochemical characteristics of the proposed waste site and of repository liner material are essential information in providing key model parameters needed to determine the mechanisms and rates for the potential release and transport of radionuclides in the environment. A series of laboratory sorption and diffusion experiments in support of the site conceptual model were run at Los Alamos National Laboratory, USA and Institute for Nuclear Research, Romania. These experiments compared responses of cesium-137 and tritiated water as radionuclides, and included natural soil samples from Saligny site and cement liner material. Sorption experiments focused on studying the affinity of cesium-137 for soil samples from: a clay rich loess horizon, a compacted red clay horizon and a Portland type cement, proposed as liner for the repository. A Saligny synthetic water was used for the soil sorption experiments. For the cement sorption and diffusion experiments, NaCl 10mM water was necessary in order to avoid insolubility problems at the high pH (around 12) when in contact with the cement. Preliminary studies of the geochemistry for radionuclide-soil matrix and radionuclide-concrete liner interactions were obtained through the sorption and diffusion experiments. The sorption isotherms were linear for all samples with high Kd values for soil samples and low values for the cement. Mineralogical information obtained through quantitative X-ray diffraction (QXRD) analysis, for the studied soil samples were in agreement with the sorption results. The high amount of clay present in the soil samples was determined to be the main contributor for the strong sorption. Diffusion cells were prepared for comparing hydrologic response characteristics for tritiated water and cesium-137, through the measurement of diffusion coefficients as influenced by the soil and cement matrices. Tritiated water and cesium-137 diffusion coefficients were determined from the change in radionuclide concentration profile over time while diffusing through the soil or cement matrix, and were found in agreement with the values listed in literature findings.Copyright © 2003 by ASME

Crina Bucur

Papers

The Importance of Geochemical Characterization of Repository Host Horizons for Radioactive Waste Disposal: Saligny Repository Site for L/ILW, Romania